I. Field of the Invention
This invention relates to section mills for cutting through well casing by rotation of the tool and, in particular, to a section mill with multiple sets of cutting blades which are independently and successively engageable such that milling can continue without retrieving the tool as a set of cutting blades becomes non-functional.
II. Description of the Prior Art
A variety of cutting tools are utilized in the development and completion of wells, specifically to cut through or sever the well casing. Casing cutters may be used to form a transverse cut of the casing for removal of the well head or a section of the well casing. Milling tools are used to cut a hole through the casing for diverting the well bore or forming a horizontal bore. In most instances, the cutting tool is rotated to allow the cutting elements to cut through the casing. Obviously, the cutting elements will be dulled after prolonged cutting requiring replacement of the elements. If the severing or diversion operation has not been completed, the cutting tool must be retrieved from the hole to replace the cutting elements.
Typical prior known section mills include one set of cutter elements pivotably mounted within openings in the outer sleeve. The cutter elements engage an axially displaceable inner mandrel which is biased downwardly by a spring. The mandrel includes a sloped surface which moves beneath the cutter elements to force the elements radially outwardly as the mandrel is axially displaced against the force of the spring. The mandrel is displaced by hydraulic pressure. Fluid is pumped to the bottom end of the mandrel to force the mandrel upwardly within the outer sleeve in the nature of a piston within a cylinder. As hydraulic pressure is increased the cutting elements will be forced radially outwardly. Rotation of the tool will cause the cutting elements to cut against the casing. However, after a period of time the cutting elements will lose their ability to cut away the casing material particularly if thick casing is encountered. In such a situation the cutting tool must be retrieved from the hole to replace the cutting elements resulting in lost time and increased expense.